Expanding our knowledge of Mt Isa to a third dimension

George Gibson, Paul Henson, Avon McIntyre and Narelle Neumann

Australian researchers have been working together to interpret the geology
and prospectivity of the Mount Isa Western Succession.

Since March 2002, a collaborative project to improve our understanding of
the 3D crustal architecture and mineral systems of the Mount Isa Western Succession
has engaged researchers from Geoscience Australia, universities, industry
and other government agencies. The project has been carried out under the
auspices of the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC).

The project gave high priority to building of a 3D structural model of the
Mount Isa region, incorporating stratigraphy, major unconformity surfaces,
fault geometries, basin shape and mineral deposit locations. Complementary
investigations into the geodynamic setting, origin and timing of events that
may have influenced or controlled fluid flow in the region helped the researchers
achieve:

a comprehensive and internally consistent 3D interpretation of basin
architecture and regional structure that allows mineral alteration patterns
to be better understood in terms of fault geometries, sequence stratigraphy
and other potential fluid pathways

better understanding of the geological and tectonic processes that
controlled and influenced basin geometry and fluid flow

improved temporal constraints on the timing of these processes and
the events that gave rise to them.

The project built on previous Geoscience Australia successes with NABRE (North
Australian Basins Resource Evaluation) and external partners, such as the
AMIRA P552 Fluid Flow Modelling project. It involved collaboration with the
Queensland Department of Natural Resources, James Cook University, Melbourne
University, the University of Western Australia, CSIRO, and two exploration
companies currently active in the region, Xstrata Copper and Zinifex. The
University of Newcastle provided further geological input and analytical data
on a contractual basis.

Following a 12-month confidentiality period, the results of the project became
publicly available from 30 April 2006 and are currently being prepared for
publication.

As might be expected of such a richly endowed mineral province, the Mount
Isa region in northern Australia (figure
1) has long been the subject of intense geological research and exploration
activity. There is a diverse range of opinion about its regional structure,
tectonic evolution and metalliferous potential. This makes mineral exploration
more difficult and fraught with uncertainty, because there is rarely the time
or resources to fully assess the wide range of interpretations.

Despite such uncertainties, tectonic setting and its role in controlling crustal
architecture and the pattern of fluid flow are widely perceived to be important
factors in the formation of ore bodies and their metallogeny. The difficulty
for exploration companies is that the Western Succession, like so many other
Proterozoic terranes in northern Australia (including the Eastern Succession),
has been substantially modified by later deformation and post-depositional
processes. The original tectonic setting and basin architecture are no longer
obvious, and there is ongoing uncertainty about the age of mineralisation,
its relationship to major structures, and whether these structures first became
active before or after mineral deposition.

Marrying basin analysis with regional structural studies was deemed the most
effective means of differentiating between pre-, syn- and post-depositional
fault movements, although this is seldom enough to narrow the range of potential
exploration targets. Details of possible source rocks and traps are also required.

More importantly, the researchers thought it desirable to have all elements
of the mineralising system brought together in a single 3D representation,
permitting better visualisation of the problem and more effective discrimination
between competing geological interpretations. The 3D environment provides
a much less forgiving test bed for subsurface geological interpretation than
was ever possible from traditional 2D geological maps and cross sections.
Successful exploration now demands a more integrated approach to the problem
of target selection, requiring an understanding of the entire mineralising
system. This has been the guiding philosophy and visionary goal behind the
current research program:

… through a better understanding of the geological controls on
existing sediment-hosted Pb–Zn–Cu deposits in the Western Succession
derive the key geological parameters that will increase the predictive ability
of the exploration industry to locate blind or as yet undiscovered mineral
deposits beneath cover elsewhere in northern Australia.

Figure 1. Study area in relation to major
tectonic elements in the Mount Isa region. (Larger
image [GIF 324.0kb])

Project scope

The previous Geoscience Australia NABRE study in the Western Succession largely
concentrated on elucidation of basin architecture in the Calvert (1730–1670
Ma) and Isa (1670–1595 Ma) superbasins. (See the event chart of Neumann
et al for a more detailed geochronology of these two structural entities.)

Figure 2. Gravity response upwardly continued
from 66 to 220 kilometres to produce the ‘Barramundi worm’ as
determined by Hobbs et al (2000). Note major Pb-Zn mineral deposits located
along the trace of this feature which is thought to represent a continent-scale
boundary between crustal blocks of contrasting density. (Larger
image [JPG 587.1kb])

These studies placed less emphasis on the older underlying Leichhardt Superbasin
(1790–1740 Ma) even though it was recognised that major faults dating
from that period had the potential to exercise an important control on the
geometry and location of future, successor basins. The new study was consequently
directed at improved understanding of the architecture of the older Leichhardt
Superbasin and how this interacted with younger basins to produce the distribution
of sedimentary facies and magmatic rocks preserved in the Western Succession
today.

Particularly important in this context was an attempt to recognise rock sequences
that may have served as potential source and trap rocks for metals. No less
important was a study of fault geometry and distribution, and how these may
have influenced fluid flow and the migration of mineralising fluids through
the rock pile.

Only part of the Western Succession was investigated in detail, with the
bulk of the research being conducted in the northern part of the Leichhardt
River Fault Trough and the southern part of the Lawn Hill Platform (figure
1). As such, the study area straddled the Mt Gordon Fault Zone, a prominent
geophysical and geological feature (figure
1), as well as a significant length of the ‘Barramundi worm’
(figure
2). The latter had already been identified in upwardly continued regional
gravity datasets (Hobbs et al 2000), and appeared to serve as an important
locus for several mineral deposits at Century and Lady Loretta (figure
1). The Century Pb–Zn deposit formed part of a PhD project carried
out at James Cook University under the banner of the pmd*CRC.

Objectives, organisation and deliverables

To achieve its visionary goal, the project was set up to deliver:

3D Gocad model of basin architecture

improved understanding of the geodynamic setting and crustal architecture

more tightly constrained space–time plot and event chart

metamo rphic map and improved diagenetic/burial history for region

timing and age of fault movement with respect to depositional and
post-depositional histories and related mineralisation

pilot study in use of remote sensing as a means of determining the
distribution of particular mineral species or parageneses (e.g. muscovite-
versus phengite-dominated rocks) or alteration styles (e.g. silicification)
associated with mineralisation (see article by Oliver and van der Wielen in
this issue of AusGeo News).

To meet its objectives, the project was managed as a series of modules with
specific outcomes and deliverables. These included:

3D basin architecture underpinned by interpretation of
the potential field data (gravity and aeromagnetics) and structural analysis
undertaken in a number of key localities (e.g. Bull Creek, Barr Hole, Mellish
Park, Lake Julius). Construction of the 3D Gocad model (figure
3) is at the core of this module, although it also incorporated a study
of faults that may have been active at critical times during basin formation
and subsequent inversion and evolution. The geometry and age of these faults
were determined through a combination of structural cross-sections, map
patterns and field studies. Thermobarometric data (Kubler indices and white
mica b dimensions) were employed to constrain the depth of tectonic burial
and related metamorphic history of the basin.

3D isopach map and identification of sedimentary depocentres
to constrain basin shape and the location of growth faults (figure
3). This module combined mapped sedimentary thicknesses obtained from
the 1:100 000 sheets with sequence stratigraphy of detailed measured sections
obtained from many of the same key localities used to constrain the deformational
history. In view of the detailed work previously undertaken by NABRE (Southgate
et al 2000) on the Calvert and Isa supersequences, the focus of this module
was on the older underlying units (Myally and Quilalar supersequences) that
formed the original rift template.

Mineral index maps and numerical modelling were employed
to assess the extent and degree of fluid flow accompanying basin formation
and its subsequent inversion. A critical component of this module has been
the processing of remotely sensed LANDSAT, ASTER, HYPERION and radiometric
data to identify the possible footprints of fossil hydrothermal systems.
Large numbers of PIMA analyses were undertaken in support of this module,
in order to properly calibrate and ground-truth the remotely sensed data.
Cross-referencing with the results of module 1 has been carried out to ascertain
whether there is any relationship between the inferred hydrothermal hotspots
and regionally significant structures such as faults, stratigraphic boundaries
or high-strain corridors. As a test of how basin architecture and fault
geometry may have controlled or influenced fluid flow and paleo-temperatures
during basin inversion, a number of numerical simulations were run in Flac3D,
based on simplified geological models.

An extended and revised space–time and event plot
was necessitated by improved chronostratigraphic control on the age of sedimentary
deposition (see the article by Neumann in AusGeo News 78), as well as a
better understanding of the present-day 3D basin architecture and the kinematic
framework that gave rise to this architecture. Particularly important in
this regard is a large component of U–Pb SHRIMP dating of detrital
zircon populations from different parts of the stratigraphy. The results
of this work provide important constraints on depositional ages, as well
as the provenance of the sediments. Several granites from the Sybella Batholith
were also selected for zircon analysis to refine their ages in relation
to both the depositional history and the tectonic evolution of the basin.